Pressure relief devices

ABSTRACT

In combination with a heat exchanger of the tube-in-shell type in which water/steam flows in the tubes and a liquid metal such as sodium flows in the closed shell and over the tubes, a safety expedient comprises a duct communicating with the lower region of the shell well below the operating level of liquid metal therein, a storage vessel to which said duct leads and which is disposed at a lower level than that of said shell, and a pressure relief device disposed in said duct and normally closing it but capable of opening said duct on the occurrence of an overpressure in said shell such as will be caused by evolution of hydrogen from reaction between the liquid metal and water/steam on failure of a heat exchange tube. Thus the overpressure aided by the force of gravity will force the liquid metal out of the shell and into the storage vessel. A particularly suitable form of pressure relief device is described. Isolation of the heat exchanger from liquid metal, and additionally from water/steam if desired, can be initiated by evolution of hydrogen in said shell.

United States Patent [721 Inventors Derek Taylor Knutsford; Michael JohnWatts, Culcheth, Warrington, both of, England 1211 Appl. No. 840,7641221 Filed July 10, 1969 [45] Patented June 8, 1971 I731 Assignee UnitedKingdom Atomic Energy Authority London, England [32] Priority July 18,1968 1331 Great Britain [31] 34421/68 [54] PRESSURE RELIEF DEVICES 4Claims, 3 Drawing Figs.

[52] U.S.Cl 165/134, 122/32, 165/158, 137/68,137/69,l37/71, 176/37,220/89A [51] Int. Cl F281 9/00 [501 Field of Search 165/134, 158;122/32; 137/68,69, 71;220/89 A; 176/37 [56] References Cited UNITEDSTATES PATENTS 2,079,164 5/1937 Glab 137/69X I I I I 8? 9 l 1... T 5/ II l 2,128,039 8/1938 Cibulka 137/69 2,895,492 7/1959 Bell 137/683,398,789 8/1968 Wolowodiuk et al 165/134 3,438,431 4/1969 Dreyer et al.165/134 Primary Examiner-Albert W. Davis, Jr. Anorney-Larson, Taylor andHinds ABSTRACT: In combination with a heat exchanger of thetube-in-shell type in which water/steam flows in the tubes and a liquidmetal such as sodium flows in the closed shell and over the tubes, asafety expedient comprises a duct communicating with the lower region ofthe shell well below the operating level of liquid metal therein, astorage vessel to which said duct leads and which is disposed at a lowerlevel than that of said shell, and a pressure relief device disposed insaid duct and normally closing it but capable of opening said duct onthe occurrence of an overpressure in said shell such as will be causedby evolution of hydrogen from reaction between the liquid metal andwater/steam on failure of a heat exchange tube. Thus the overpressureaided by the force of gravity will force the liquid metal out of theshell and into the storage vessel. A particularly suitable form ofpressure relief device is described. Isolation of the heat exchangerfrom liquid metal, and additionally from water/steam if desired, can beinitiated by evolution of hydrogen in said shell.

SHEET 2 OF 3 PATENIEU JUN 8 |97l Q a T. w r E 5 f vv mm F A Q QN AN Q Wwv B m Q R m QM 7L mm w Q Q mm wv Z. m m vv mm mm a mm. INA Q N w m mmwwm EL mm PRESSURE RELIEF DEVICES BACKGROUND OF THE INVENTION Thisinvention relates to a safety expedient applicable where two reactingsubstances, one a liquid metal (such as sodium) and the otherwater/steam, flow in heat exchange relationship with the interpositionbetween the substances of a barrier material, such as the walls of anest of tubes extending into and out of a closed vessel, the water/steamflowing through the tubes and the liquid metal flowing over the tubes.Local failure of the barrier material will permit contact between theliquid metal and water/steam, the resulting violent reaction producinghydrogen and causing a pressure rise in the closed vessel. The pressurerise, if not relieved, can cause further failures of the barriermaterial, which leads to further reaction and further pressure rise, andan explosion ultimately may occur. In addition to relieving thepressure, it is desirable to be able to interrupt the flow of at leastone of'the substances.

Hitherto, in a sodium/water heat exchange system of the tube-in-shelltype employing U-shaped tubes and in which, to avoid having the tubeplates and tube/tube plate welds in a sodium environment, an inert gasspace is provided above the sodium at each end of the tubes, pressurerelief means of the bursting disc-type has been provided to relieve highpressure in such gas spaces when caused by the hydrogen evolved in thesodium/water reaction on failure of one or more tubes. Such pressurerelief has been augmented by a separately controlled arrangement wherebythe steam/water can be removed from the tubes by allowing the steam toflash off, and the water feed to the tubes can be interrupted.

SUMMARY OF THE INVENTION The present invention deals with thesodium/water reaction problem by employing a new concept, namely thatthe pressure rise following from such reaction is employed, togetherwith gravity, to remove sodium from the reaction zone, thus avoiding theexplosive hazard which could arise from following the expedient hithertoprovided in the situation where the arrangement for removing steam/waterfrom the tubes failed to operate, or operation was not effected in time.

Hence, according to the invention, in combination with a liquidmetal/water or steam heat exchanger of the tube-inshell type intended tooperate so that water/steam flows in the tubes and liquid metal flows inthe closed shell and over the tubes, a safety expedient is characterizedby a duct communicating with the lower region of the shell well belowthe operating level of liquid metal therein, said duc't leading to astorage vessel disposed at a lower level than that of said shell, and apressure relief device disposed in said duct and normally closing it,whereby, on failure of a tube such that reaction between liquid metaland water/steam takes place and hydrogen is evolved, the resultingpressure rise in the shell operates the pressure relief device to openthe duct and also, together with the effect of gravity, forces liquidmetal out of the shell and into the storage vessel.

The pressure relief device preferably comprises a membrane normallypreventing flow of liquid metal in said duct and of a material inert toliquid metal, and, on the downstream side of said membrane, a rigidbacking supporting said membrane, said backing being pivotable from aposition in which said membrane closes said duct into a position openingsaid duct, said backing being held in the duct-closing position by ashear pin designed to fail, together with said membrane, at apredetermined pressure of liquid metal whereupon 'the pressure of liquidmetal opens the duct so that the liquid metal can flow therealong.

There is preferably provided a second membrane, unsupported, downstreamof said membrane and backing, to provide a holdup for said metal onpremature opening or malfunctioning of the pressure relief deviceconstituted by said membrane and backing, and also providing aninterspace for the detection of any leakage from said pressure reliefdevice.

The said membrane or both of them are preferably of nickel sheetmaterial.

There are preferably two shear pin locations, accessible from outsidesaid duct, although one shear pin only is employed in operation,allowing a shear pin to-be removed and replaced on routine inspection'byemploying a temporary shear pin in the alternative location.

DESCRIPTION OF THE DRAWINGS A constructional example embodying theinvention will now be described with reference to the accompanyingdrawings, wherein FIG. I is a diagrammatic flow diagram,

FIG. 2 is a fragmentary side view in medial section, and

FIG. 3 is an end view in section on line III-III of FIG. -2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIGS.2 and 3 illustrate the provis'ion of a pressure relief device 1 in aduct 2 for liquid metal, such as sodium, at an elevated temperature('e.g. 350 C.), the device 1 forming part of a safety expedient,illustrated in FIG. 1, for a liquid metal heated steam generator,reheater or superheater, such for example as is required for apower-producing, liquid metal cooled, fast breeder nuclear reactor suchas the Prototype Fast Reactor (P.F.R.), now being constructed atDoun'rea'y, Scotland. A typical sodium heated steam generator orsuperheater, which is of the tube-in-shell type, is showndiagrammatically in FIG. 1 and designated generally by the referencenumeral 3, and consists of a shell embodied by a pressure vessel 4containing heat exchange tubes, indicated diagrammatically bydot-and-dash lines 5, of U-configuration and terminating at their endsin water or steam headers 6, 7 respectively. The tube/tube plate weldsare isolated from liquid metal by inert gas (e.g. argon) spaces 8, 9above the liquid metal levels I0, 11 and a partition 12 isolates the gasspaces 8, 9 from one another. An inlet 13 to the pressure vessel isprovided for sodium in a secondary circuit, which sodium flowsdownwardly then upwardly over the U-tubes'S, leaving the pressure vessel4 at outlet 14. Each secondary circuit includes one side of heatexchangers in the primary sodium circuit of the reactor, a circulatingpump, and a steam generator, superheater and reheater.

A further outlet 15 is provided at the lower end region of the'pressurevessel 4 with which the duct 2 communicates, the latter being normallyclosed by the pressure relief device 1. Although shown in FIG. 1 in'thelower end region of the vessel 4, the outlet 1 5 for the duct 2 can withadvantage be at the lowest point of the vessel 4 assuming thatsufficient room exists for the subsequently-described storage vessel ata lower level. The device is intended to operate should a heat exchangetube 5 fail during operation, therebyallowing water or steam to comeinto contact with the flowing sodium. In this event, a rapid reactionwill ensue and hydrogen will be generated which will rapidly raise thepressure within the pressure vessel and,added to the load on thepressure relief device which is manifest by the pressure drop in thesodium and the sodium head itself, will operate the pressure reliefdevice 1, open the outlet 15 and force the sodium, aided by gravity,from the pressure vessel 4 into the duct 2 to flow along duct 2 to -astorage vessel 16 designed to accept more than the full quantity ofsodium in the particular heat exchanger concerned. The storage vessel 16and that part of the duct -2 downstream of the pressure relief device 1is normally at atmospheric pressure. It is arranged by conventionalfault circuitr-y that when the pressure relief device 1 operates, orwhen evolution of hydrogen in the vessel 4 is detected by suitablemonitoring equipment, the nuclear reactor is shut down and the affectedheat exchanger is isolated from sodium. It can be additionally arrangedthat the steam is flashed off from the steam circuit of the affectedheat exchanger and where the heat exchanger is a steam generator, thegenerator is isolated from water. Such additional measures provide asafeguard which augments the'main safeguard provided by removing thesodium from the vessel 4 and preventing the entry of further sodium tothe vessel 4 from the said secondary of circuit by the said isolation.

Referring again to FIGS. 2 and 3, the pressure relief device 1 in theduct 2 comprises a sheet nickel disc membrane 17 whose peripheral edgeis sealingly held between a pair of annular joint rings 18 also ofnickel, the joint rings 18 being themselves sealingly held betweenflanges 19, 20 of two portions 21, 22 respectively of tubes constitutingthe duct 2. The flanges 19, 20 are held in sealing engagement againstthe rings 18 by rings 23, 24 which engage nonsealing faces of theflanges 19, 20, and by a plurality of bolts 25 extending between therings 23, 24-. Additionally there is an annular soft metal backing ring26 in engagement with the outer circumferential surfaces of the flanges23, 24.

The flange 20 of the duct portion 22 has an inwardly projecting lug 27which is apertured at 28 to form a pivot for a backing plate 29 for themembrane 17. The pivot is formed by two spaced diametrical members 30welded to the plate 29 and pivotally mounted at one of their ends on apivot pin 3! accommodated in the aperture 28 of the lug 27. The members30 are welded at their other ends to a shear pin block 32 having tworadially extending blind holes 33, 34. The flange 20 of the duct portion22 has an inwardly extending block 35 welded to it at a positiondiametrically opposed to that of the lug 27, the inner surface of theblock 35 registering with and just clearing the outer surface of theblock 32. The block 35 and the flange 20 have registering apertures 36,37 which also register with the blind holes 33, 34 respectively, Theapertures in the flange 20 each have an upper portion of increaseddiameter and screw threaded for a cap bolt 38; a shear pin 39 with ahead 40 for engaging the increased diameter portion is employed in oneof the pairs of registering apertures and blind holes 36, 33 or 37, 34,the other being left for changeover pur poses. It is advantageous thatthe shear pin locations are accessible from outside the duct, enablingroutine inspection to be performed without dismantling any pipework. Atan axial position in the duct 2 which is clear of the pivotal arc of thebacking plate 29 and block 32 there is provided a backup dished-discmembrane 41 whose peripheral edge is trapped in sealing manner between aflange 42 welded to the interior of the duct portion 22 and a ring 43fastened to the flange 42 by bolted studs 44. O-ring seals 45 areprovided on the opposed faces of the flange 42 and ring 43 to sealagainst the membrane 41. A bellows joint 46 is interposed in the duct 2downstream of the pressure relief device 1, and in a modification, thebackup membrane 41 can be incorporated between flanges which form partof the bellows construction (not shown but of conventional type).

The characteristics of the pin 39 in shear are chosen so that the pinwill fail at a desired overpressure in the pressure vessel. When the pin39 fails, the pressure causes the membrane 17 to fail also, and theplate 29 is forced to pivot and assume an open position, therebyallowing sodium to flow along duct 2 into the storage vessel 16.Although the membrane 17 can be designed to fail at any desired pressurewithin a small percentage error, it will be appreciated thatparticularly at high temperatures when the nonnal working pressure isclose to the failure pressure, the working life of the membrane 17 wouldbe severely reduced if provided unsupported. However, when supported asdescribed, the stresses in the membrane 17 are reduced and its lifeprolonged. By employing the backing plate 29 to give rigidity to themembrane 17 and the shear pin 39 to fail at a desired overpressure, themembrane 17 (which will fail with the shear pin 39) still serves auseful purpose in that it seals the duct 2 during normal operation (itwould be extremely difficult to seal against hot sodium around theperiphery of the backing plate if this had to function as a flap valve)and also, because it forms the barrier for the hot sodium, it enablesthe material of the backing plate 29, members 30 and blocks 32, 35 to beof conventional (and hence cheaper) materials compared with the specialmaterials which would be necessary if these parts had to have good andlong term corrosion resistance in hot sodium.

The backup membrane 41 is provided to delay the discharge of the wholesodium content of the heat exchange circuit involved where, due to somemaloperation or unintended failure, the barrier provided by the membrane17 and backing plate 29 becomes removed prematurely and without beingcaused by a tube failure in the heat exchanger. The back up membrane 41is designed to fail subsequently to failure of the membrane 17consequent on a pressure rise in the vessel 4 due to the occurrence of asodium, steam/water reaction arising from failure of one of the heatexchanger tubes 5. The membrane 41 can also serve to provide aninterspace 47 so that, should there by unintentional leakage of sodiumpast the initial barrier, it can be detected within the interspace 47 byconventional means (not shown) and remedial measures to prevent theleakage can be taken, without losing sodium to the storage vessel 16.

The storage vessel 16 is preferably provided (not shown) with a normallyclosed, high, vent line so that hydrogen which becomes entrained in thesodium or reaches the tank 16 after the sodium has reached it, can bevented to atmosphere. The vent line top has a simple pressure-frangiblediaphragm to allow the venting, and conventional means are provided forremoving any entrained sodium from the gas to be vented.

We claim:

- 1. In combination with a heat exchanger of the tube-in-shell type foroperating so that water/steam flows in the tubes and liquid metal flowin the closed shell and over the tubes a safety expedient comprising aduct communicating with the shell at the lower end thereof, a storagevessel to which said duct leads and which is disposed at a lower levelthan that of said shell and a pressure relief device disposed in saidduct and normally closing it, said pressure relief device comprising aclosure membrane, means supporting and sealing the closure membrane inthe duct, said closure membrane normally preventing flow of liquid metalin said duct and being of a material inert to liquid metal, and, on thedownstream side of said closure membrane, a rigid backing member bearingagainst and supporting said closure membrane, said rigid backing memberbeing pivotable from the position supporting said closure membrane to aposition leaving the duct open on failure of the closure membrane, saidrigid backing member being held in the position supporting the closuremembrane by a shear pin designed to fail, together with said closuremembrane, on a pressure rise in the heat exchanger shell such as wouldbe caused by evolution of hydrogen from reaction between the liquidmetal and water/steam on failure of a heat exchanger tube, therebyopening the duct on failure of the closure membrane and pivoting of therigid backing member to allow flow ofliquid metal from the heatexchanger shell through the duct into the storage vessel.

2. A safety expedient according to claim 1 whereby a second membrane isprovided without a rigid backing member downstream of said closuremembrane and backing member, means being provided supporting and sealingsaid second membrane in the duct, said second membrane being provided toact as a holdup for liquid metal on premature opening or malfunctioningof the pressure relief device constituted by said closure membrane andthe rigid backing member, said second membrane being designed to failand allow the flow of liquid metal along the duct subsequent to failureof said closure membrane and release of the backing member by failure ofthe shear pin holding the backing member in the position supporting theclosure membrane consequent on a pressure rise in the shell due to theoccurrence of a liquid metal steam/water reaction on failure of a heatexchanger tube, said second membrane also providing an interspace forthe detection of any leakage from said pressure relief device.

3. A safety expedient according to claim 2, wherein the liquid metal issodium and said closure membrane is of nickel.

4. A safety expedient according to claim 1, wherein there are two shearpin locations, accessible from outside said duct, although one shear pinonly is employed in operation, allowing a shear pin to be removed andreplaced on routine inspection by employing a temporary shear pin in thealternative iocation.

1. In combination with a heat exchanger of the tube-in-shell type foroperating so that water/steam flows in the tubes and liquid metal flowin the closed shell and over the tubes a safety expedient comprising aduct communicating with the shell at the lower end thereof, a storagevessel to which said duct leads and which is disposed at a lOwer levelthan that of said shell and a pressure relief device disposed in saidduct and normally closing it, said pressure relief device comprising aclosure membrane, means supporting and sealing the closure membrane inthe duct, said closure membrane normally preventing flow of liquid metalin said duct and being of a material inert to liquid metal, and, on thedownstream side of said closure membrane, a rigid backing member bearingagainst and supporting said closure membrane, said rigid backing memberbeing pivotable from the position supporting said closure membrane to aposition leaving the duct open on failure of the closure membrane, saidrigid backing member being held in the position supporting the closuremembrane by a shear pin designed to fail, together with said closuremembrane, on a pressure rise in the heat exchanger shell such as wouldbe caused by evolution of hydrogen from reaction between the liquidmetal and water/steam on failure of a heat exchanger tube, therebyopening the duct on failure of the closure membrane and pivoting of therigid backing member to allow flow of liquid metal from the heatexchanger shell through the duct into the storage vessel.
 2. A safetyexpedient according to claim 1 whereby a second membrane is providedwithout a rigid backing member downstream of said closure membrane andbacking member, means being provided supporting and sealing said secondmembrane in the duct, said second membrane being provided to act as aholdup for liquid metal on premature opening or malfunctioning of thepressure relief device constituted by said closure membrane and therigid backing member, said second membrane being designed to fail andallow the flow of liquid metal along the duct subsequent to failure ofsaid closure membrane and release of the backing member by failure ofthe shear pin holding the backing member in the position supporting theclosure membrane consequent on a pressure rise in the shell due to theoccurrence of a liquid metal steam/water reaction on failure of a heatexchanger tube, said second membrane also providing an interspace forthe detection of any leakage from said pressure relief device.
 3. Asafety expedient according to claim 2, wherein the liquid metal issodium and said closure membrane is of nickel.
 4. A safety expedientaccording to claim 1, wherein there are two shear pin locations,accessible from outside said duct, although one shear pin only isemployed in operation, allowing a shear pin to be removed and replacedon routine inspection by employing a temporary shear pin in thealternative location.